Led Emitting Diode

Rea Jhoy LastimosaChristian Joseph Garniel

Christian Lara

Introduction

Light can be emitted from a semiconductor material as a result of electron-hole recombination.

However, materials capable of emitting such light do not glow at room temperature because the concentrations of thermally excited electrons and holes are too low to produce discernible radiation.

On the other hand, an external source of energy can be used to excite electron-hole pairs in sufficient numbers such that they produce large amounts of spontaneous recombination radiation, causing the material to glow or luminesce.

A convenient way of achieving this is to forward bias a p-n junction, which has the effect of injecting electrons and holes into the same region of space; the resulting recombination radiation is then called injection electroluminescence

Electroluminescence

• The spontaneous emission of light due to the radiative recombination from within the diode structure is known as electroluminescence (EL).• The term electroluminescence is used when

the optical emission results from the application of an electric field.

Electroluminescence

• The light is emitted at the site of carrier recombination which is primarily close to the junction, although recombination may take place through the whole diode structure as carriers diffuse away from the junction region.• The amount of radiative recombination and the emission

area within the structure is dependent upon the semiconductor materials used and the fabrication of the device.

• When a forward bias nearly equals or exceeding the bandgap voltage there is conduction. (eV > Eg)

• At high injection carrier density in such a junction there is an active region near the depletion layer that contains simultaneously degenerate populations of electrons and holes.

• The injection carrier may be largely electrons injected into the p-n region because of their larger mobility.

Radiative recombination in a forward-biased p-n junction

• The increased concentration of minority carriers in the opposite type region in the forward-biased p-n diode of direct-bandgap materials leads to the radiative recombination of carriers across the bandgap.

• The normally empty electron states in the conduction band of the p-type material and the normally empty hole states in the valence band of the n-type material are populated by injected carriers which recombine with the majority carriers across the bandgap.

• The energy released by this electron-hole recombination is approximately equal to the bandgap energy Eg.

Light-emitting diodes

• Internal quantum efficiency• Extraction efficiency• External quantum efficiency• Power conversion efficiency• Responsivity• Spectral distribution

Light-emitting diodes (LEDs)

• The light output of an LED is the spontaneous emission generated by radiative recombination of electrons and holes in the active region of the diode under forward bias.• The semiconductor material is direct-bandgap to

ensure high quantum efficiency, often III-V semiconductors.

LEDs

• An LED emits incoherent, non-directional, and unpolarised spontaneous photons that are not amplified by stimulated emission.• An LED does not have a threshold current.

It starts emitting light as soon as an injection current flows across the junction.

Internal quantum efficiency

• The internal quantum efficiency η int of a semiconductor material: the ratio of the radiative electron-hole recombination coefficient to the total (radiative and nonradiative) recombination coefficient.• This parameter is significant because it determines

the efficiency of light generation in a semiconductor material.

• Recall that the total rate of recombination = r n p [cm-3 s-1]• If the recombination coefficient r is split into a

sum of radiative and nonradiative parts, r = rr + rnr, the internal quantum efficiency is:

ηint = rr / r ηint = rr / (rr + rnr)

Recombination lifetimes

• The internal quantum efficiency may also be written in terms of the recombination lifetimes as is inversely proportional to r.• Define the radiative and nonradiative

recombination lifetimes tr and tnr 1/t = 1/tr + 1/tnr

• The internal quantum efficiency is then given by rr/r = (1/tr)/(1/t)

η int = t / tr = tnr / (tr + tnr)

*Semiconductor optical sources require η int to be large (in typical direct bandgap materials tr ≈ tnr).

• The radiative lifetime for bulk Si is orders of magnitude longer than its overall lifetime because of its indirect bandgap (electron momentum mismatched). This results in a small internal quantum efficiency.• For GaAs, the radiative transitions are sufficiently fast

because of its direct bandgap (electron momentum matched), and the internal quantum efficiency is large.

Electroluminescence in the presence of carrier injection• The internal photon flux Ф(photons per second),

generated within a volume V of the semiconductor, is directly proportional to the carrierpair injection rate R (electron-hole pairs/cm3-s).• The steady-state excess-carrier concentration Д n

= Rt, where t is the total recombination lifetime (1/tr = 1/ tr + 1/ tnr ).

• The injection of RV carrier pairs per second therefore leads to the generation of a photon flux Ф = η int RV photons/s.

Ф = η int RV = η int V Д n/t = V Д n/tr

Or Ф = η int V (i/e)/V = η int i/e

LED efficiencies

• Internal quantum efficiency η int - only a fraction of the electron-hole recombinations are radiative in nature• Extraction efficiency η e – only a small fraction of

the light generated in the junction region can escape from the high-index medium• External quantum efficiency η ext = η e η int (can be

measured from the responsivity R = Po/i)

• Power-conversion (wall-plug) efficiency η c – efficiency of converting electrical power to optical power (ηc ≤ η ext)•Coupling efficiency η couple – only a fraction of the light emitted from the LED can be coupled (e.g. to an optical fiber)